Method for improving the rate of a plasma enhanced vacuum treatment

ABSTRACT

For increasing the rate with which a workpiece is treated in a plasma enhanced chemical vapor deposition method and thereby lowering for coating treatment exposure of the coating to ion impact, there is maintained a non-vanishing dust particle density along the surface to be treated with a predetermined density distribution along this surface. The density distribution may be controlled by appropriately applying a field of force substantially in parallelism to the surface to be treated and acting on the dust particles entrapped in the plasma discharge.

This application is a continuation of application Ser. No. 08/903,000,filed Jul. 30, 1997, now abandoned which is a divisional of applicationSer. No. 08/237,432, filed May 3, 1994, now U.S. Pat. No. 5,693,238.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to a method and vacuum plasma chamberfor improving the treatment rate of a plasma enhanced vacuum treatmentof a surface of workpiece.

We thereby define the treatment rate of such vacuum treatment of asurface as the amount of material per time unit which is removed orwhich is deposited from or on that surface respectively.

Thereby the present invention is especially directed on such animprovement for reactive plasma etching, reactive plasma sputtercoating, reactive ion plating or and especially for plasma enhancedchemical vapor deposition coating known as PECVD.

Especially for semi-conductor production where plasma enhanced treatingprocesses are used, thereby especially plasma enhanced coatingprocesses, it is of tremendous importance to prevent that the workpiecesurface being treated is contaminated by contamination particles,especially by dust or powder particles. Under plasma enhanced coatingtreatments we especially understand plasma enhanced reactive coatingtreatments as e.g. plasma enhanced chemical vapor deposition known asPECVD treatments, and thereby especially enhanced by RF plasma atfrequencies from 1 to 100 MHz.

Preventing such contamination is a most serious problem to be resolvedfor such manufacturing processes.

2. Description of Prior Art

The attempts to resolve this problem up to now were directed on tryingto minimize the generation of dust or powder during such treatmentprocesses. Nevertheless, such dust or powder generation may notcompletely be prevented. Therefore, the attempts were additionallydirected on removing the residually generated dust as efficiently aspossible from that plasma discharge area which is significantlycontributing to the treatment. This means that one tried to reach in theplasma discharge space there, where the plasma discharge issignificantly contributing to the treatment, a dust- or powder-freestate. Reference is made to the EP-A-425 419 and the EP-A-453 780.Attention is further drawn to the following prior art: EP-A-0 419 930(corresponding to JP-A-3 153 885 and U.S. Pat. No. 5,102,496), EP-A-0453 780 (corresponding to JP-A-5 074 737), EP-A-0 425 419 (correspondingto CA-A-2 024 637 and JP-A-3 147 317), EP-A-0 272 140 (corresponding toU.S. Pat. No. 5,000,113, JP-A-63 246 829, U.S. Pat. Nos. 4,872,947,4,892,753, 4,960,488, 5,158,644).

SUMMARY OF THE INVENTION

It is an object of the present invention to improve the treatment rateand thereby especially, for a plasma enhanced coating deposition vacuumprocess, the coating rate without negatively affecting surface qualityof the surface being treated, but thereby even improving its quality.

This object is resolved by a method for improving the treatment rate ofa plasma enhanced vacuum treatment of a surface of a workpiece whichcomprises the step of generating substantially along the surface to betreated in the plasma dust with a predetermined distribution of itsdensity.

The basis of the present invention is the recognition made by theinventors that dust or powder particles in a plasma dischargesignificantly increase the coupling degree of electrical energy to theplasma. Due to this phenomenon the treatment rate and especially for alayer deposition treatment, the deposition rate is significantly risen.Additionally, such dust or powder particles in the plasma discharge leadto improved coating quality as e.g. to improved characteristics of thetensions within the deposited film or layer and of its fineness. This,nevertheless, is only true as long as it may be prevented that powder ordust particles accumulate on the surface being treated.

A typical vacuum range of operating pressure is between 10⁻² mbar and 10mbar, thereby preferably between 10⁻¹ mbar and 1 mbar.

Although the object to be fulfilled will also be resolved e.g. forreactive sputter etching treatments, it shall especially be resolved forcoating processes, thereby especially for reactive plasma enhancedprocesses and especially RF plasma enhanced reactive coating processes,so-called RF-PECVD processes.

Although especially directed on the RF-PECVD treatments, the inventivemethod may principally also be used for DC or AC plasma treatment or forhybrid forms with AC+DC plasma.

In contrary to the customary approaches, namely to remove powder or dustas completely as possible from the plasma discharge area which isaffecting the surface treatment, the present invention maintains in awell controlled manner the powder trapped in the plasma discharge so asto reach the advantages with respect to treatment rate and treatmentquality mentioned above. Additionally, the density of the dust or powderentrapped in the plasma discharge is maintained below a predeterminedvalue at which value powder or dust deposition on the surface beingtreated would start.

Thus, inventively, the number of dust particles per volume unit and/orthe largeness of dust particles and thus generally the dust density andits distribution is controlled especially not to vanish. By such controlpredetermined density and density distribution are achieved which havebeen found as optimal for a specific treatment process considered duringpre-experiments. Thus, prevailing dust is exploited and not just removedas completely as possible.

It is a second object of the present invention to control dust densityin a plasma discharge. This is reached by a method for reducing dustdensity in a plasma discharge space with a plasma to which a workpiecesurface to be treated is substantially uniformly exposed, whichcomprises the step of applying substantially parallel to the surface tobe treated and across said surface in said plasma a field of forceacting on dust particles being trapped in said plasma.

By generating such a field of forces and by adjusting such field asconcerns its local extent, its distribution and strength, a desiredamount of powder or dust is removed from that plasma discharge spacewhich is significantly contributing to the surface treatment. Theremoved powder or dust is primarily conveyed into a space which is lesssignificantly affecting. the treatment and from that space the dust orpowder may further be removed whenever necessary.

As a preferred embodiment, there is thus proposed a method for improvingthe treatment rate of a plasma enhanced vacuum treatment of a surface ofa workpiece, which comprises the steps of generating substantially alongthe surface to be treated. in the plasma, dust with a predetermineddistribution of its density and generating said predetermined density byapplying substantially parallel to the surface and transversallytherealong within the plasma a field of force acting on the particles ofthe dust, predominant parts thereof being trapped in the plasma.

It is a further object of the invention to control dust density assimply as possible.

This is realized by generating the field of force, at least in parts, ina preferred mode predominantly by generating a gradient of pressure.Thereby, such field of force may also, additionally to the gradient ofpressure or exclusively, be generated by an electrostatic and/or by athermal gradient, the latter in the sense of exploiting thermophoresis.

Further, the primary object mentioned above is resolved by a vacuumplasma chamber with generating means for a three-dimensionallydistributed plasma discharge in a discharge space, with a workpiecesupport defining a support area for at least one workpiece to be treatedand for exposing at least one workpiece surface area to said plasmadischarge in said space which comprises force generating meansgenerating a field of force substantially parallel to and along saidsupport area and within said plasma discharge, and controlling thedensity of dust in said plasma discharge.

The primary object is resolved primarily for treatment of substantiallyflat large areal workpieces, as especially for flat active paneldisplays, which is achieved by the inventive method and plasma chamberbeing applied and construed respectively to and for such workpiecesrespectively.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be better understood and objects other thanthose set forth above will become apparent when consideration is givento the following detailed description thereof.

Such description makes reference to the annexed drawings, wherein:

FIG. 1 shows schematically in a cross-section a plasma chamber at whiche.g. the inventive method is applied;

FIG. 2a schematically shows a preferred small configuration of a vacuumtreatment arrangement, wherein preferably the inventive method isoperated;

FIG. 2b to 2 d shows, departing from the arrangement according to FIG.2a, charge and discharge cycles of plasma chambers of the arrangement,principally according to FIG. 2a;

FIG. 3 shows schematically and at plasma chambers provided at thearrangement according to FIG. 2a, door means to at least controllablyrealize a pressure between the inside of the plasma chambers and acommon chamber wherein lateral handling openings of the chambers abut orto even reach vacuum seal;

FIG. 4a to 4 e shows schematically in top view a further arrangementwith two plasma chamber staples and one load-lock chamber and itspreferred operating cycle, the invention being operated in sucharrangement as one preferred mode;

FIG. 5a to 5 d shows schematically with respect to a plasma chamberstaple as provided in the arrangement according to FIG. 2, respectively,its centralized feeding with gas (a), its centralized pumping (b), itscentralized feeding with electrical energy (c) as well as a centralizedhandling of measuring and/or adjusting signals shown by the example ofcentrally monitoring the plasma processes in the different plasmachambers by means of a central plasma emission monitor;

FIG. 6 shows schematically and in cross-section a preferred embodimentof a load-lock chamber magazine arrangement at the arrangement accordingto FIG. 2;

FIG. 7 shows schematically and in cross-section a plasma chamber as itis provided in a preferred embodiment within the plant according to FIG.2 and which is operated and construed so as to realize the presentinvention;

FIG. 8a to 8 e different preferred embodiments for generating by gassuctioning from the plasma discharge space a field of force acting ondust in the plasma discharge by which field inventively the dust densityand its distribution is controlled;

FIG. 9a to 9 g shows by means of an operating sequence a preferred to 9g operation cycle for realizing the inventive method, whereby it isensured that, irrespective of current workpiece treatment, dust istrapped in a plasma discharge and is controllably removed therefrom.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the present description, by the expression “plasma chamber”, there isunderstood an evacuatable space area wherein an independent plasmadischarge may be sustained continuously or at least sometimes pulsed, beit a DC, an AC or, mixed, an AC+DC plasma discharge, thereby especiallyan RF plasma discharge. With respect to such chambers attention isdirected to the EP-A-0 221 812 and the EP-A-0 312 447 which are bothintegrated by reference to the present description. Such a plasmachamber is in a preferred manner at least in part limited by walls.Typically in such a plasma chamber a pressure of 10⁻² mbar to 10 mbar issustained, preferably of 10⁻¹ mbar to 1 mbar.

In FIG. 1, as an example and as a preferred variant, there isschematically shown a plasma chamber 1. It comprises on its upper sidean areally extended electrode 3 which is fed by DC, AC or AC+DC electricenergy, whereby the general expression AC shall, as was mentioned,especially enclose RF signals. Under the general term AC+DC mixed feedpulsed DC and/or pulsed RF signals are also understood. Under RF afrequency band between 1 and 100 MHZ shall be understood.

At the embodiment shown in FIG. 1, the areally flat electrode 3comprises areally distributed outlet openings 5, by which a gas, whichat least comprises a reactive gas part, is fed to the plasma dischargespace PL. At the bottom 7 of the plasma chamber 1 there is provided, inthis preferred embodiment, a lifting mechanism 9 with a drivearrangement 11 for rising workpiece substrates. The lifting mechanismcomprises e.g. three or four lifting rods 13 which are simultaneouslymoved by the drive 11 and which are e.g. sealed towards ambient of theplasma chamber 1 by means of bellows 15. It is also possible to construethe rods 13 so that they seal themselves the respective openings at thebottom 7 when they are lowered.

Such a plasma chamber forms the basic device for the inventive apparatuswhich will subsequently be described, which apparatus is in a clearlypreferred manner an apparatus for performing PECVD coating ofworkpieces, but which could be also an apparatus for other vacuumtreatment processes. Thereby, the inventive apparatus is preferably anRF PECVD layer deposition apparatus.

In FIG. 2 there is schematically shown a nearly minimal configuration ofan inventive apparatus. Such apparatus comprises, as was said in itsnearly minimal configuration, a stack 20 of e.g. twenty plasma chambers1 which are stacked one above the other. The plasma chambers 1 aremerely shown in the FIGS. 2a to 2 d schematically, but are, in apreferred manner, construed from chambers, one of which was shown andexplained with the help of FIG. 1.

The plasma chambers 1 comprise each a lateral handling opening 17, whichopenings thus form together a handling opening stack. All handlingopenings communicate into a common vacuum space 23. This vacuum space 23into which the said handling openings abut from the interior of theplasma chambers forms a transport chamber 23 _(T). In this transportchamber 23 _(T) there is provided a transport arrangement 25 whichcomprises a number of horizontal supports 27 which are, in a preferredconstruction, formed as support forks. The number of horizontal supports27 foreseen in equal to the number of plasma chambers 1 which define thestack 20. The supports 27 are, as schematically shown by the arrow H,preferably synchronously, movable in horizontal direction, e.g., and asshown, in that they all are mounted to a carrier beam 29 which isdrivingly moved forth and back. By this horizontal movement workpieces,namely in a preferred mode flat areal workpieces 31, are fed through thehandling openings 17 to the plasma chambers 1 or are recovered from saidplasma chambers 1 to the transport chamber 23 _(T) as is shown fromFIGS. 2b to 2 d.

For loading all the plasma chambers 1 of the staple 20, the transportarrangement 25, according to FIG. 2a, is moved to the right hand side upto reaching the position according to FIG. 2b. Then. the liftingmechanism 9, which was shown at the plasma chamber 1 of FIG. 1, with therods 13, is lifted, so that in all plasma chambers 1 the workpieces 31are simultaneously lifted from the supports 27. This is shown in FIG. 2bschematically by the arrow V.

After lifting the workpieces 31 with the help of the lifting mechanism 9with its lifting rods 13, according to FIG. 1, and reaching relativepositioning according to FIG. 2c, the transport arrangement 25 with thesupports 27 is horizontally moved back as shown in FIG. 2c by the arrow−H and the workpieces 31 are lowered into their treating position bylowering the lifting mechanism 9 according to FIG. 1.

It is evident that the described vertical relative movement ofworkpieces 31 with respect to the supports 27 could also be realized bysynchronously lowering the supports 27 within the chambers 1 or, forrecovering the workpieces, by synchronously lifting said supports 27,thereby depositing the workpieces in the chambers 1 for their treatmentonto stationary supports.

In the nearly minimal configuration of the treatment apparatus whichcomprises, as was mentioned, a vacuum area with the plasma chamber stack20 and further a transport area or space 23 _(T), wherein the transportarrangement 25 is arranged and moved, further comprises, according toFIG. 2a, a load-lock chamber 30. As schematically shown, the load-lockchamber 30 is locked towards the transport area 23 _(T) by a firstload-lock gate 32 and towards ambient by a further load-lock gate 34.Within the load-lock chamber there is provided a magazine arrangement 36for buffering workpieces still to be treated and/or which have alreadybeen treated.

To be able not only to server the stack 20 of plasma chambers 1, butalso the magazine arrangement 36 within the load-lock chamber, thetransport arrangement 25 is not only shiftable in the horizontaldirection H or −H, but is additionally drivingly rotatable around avertical axis as shown at ω. Thereby, the supports 27 are rotated intoserving positions for the load-lock chamber 30 and the magazinearrangement therein and into serving position for the handling openingsof the plasma chambers 1.

As was mentioned above, the plasma chambers 1 of the stack 20 arepreferably construed so as to perform PECVD treatments. Depending on thetreatment process to be performed therein, the handling openings 17 ofthe plasma chambers 1 remain open towards the transport are 23 _(T)during workpiece processing within the chambers 1, or there is justinstalled a pressure stage between the inside of the plasma chambers 1and the transport are 23 _(T), across which a pressure differencebetween inside of the plasma chambers and the transport area 23 _(T) maybe installed or the plasma chambers 1 are closed in a vacuum tightmanner during the said workpiece treatment. If processing is PECVD, thenthe openings 17 are closed.

In FIG. 3 there are schematically shown two preferred modes ofrealization for shutting the handling openings 17 during workpiecetreatment in a vacuum tight manner or just for installing a pressurestage with respect to said transport are 23 _(T). A movable slide 38,movable in vertical direction as shown by the double arrow ±V, isprovided which, rastered, according to the handling openings 17 alongthe stack 20, is provided with handling feed-through openings 39. Thehandling feed-through openings 39 are positioned by respective moving ofthe slide 38, according to FIG. 3b, to be aligned with the handlingopenings 17 of the plasma chambers 1, when handling of workpiecesthrough the handling openings 17 is to be performed. In this position,the supports 27 may serve the plasma chambers through the feed-throughhandling openings 39.

The slide 38 further comprises horizontally driven, movable shut-offplates 41 which are e.g. driven by capsulated driving rods and drives43.

For shutting-off the treating areas within the chambers 1, the slide 38is vertically moved into the position as shown in FIG. 3a. Then, theshut-off plates 41 are driven, according to that figure, to the righthand side, so as to shut off the handling openings 17 of the plasmachambers 1 in a vacuum tight manner or so as to just install between thetransport area 23 _(T) and the said treatment areas within said chambers1 a pressure stage, whereacross pressure difference may be installed.

Departing from the nearly minimal configuration of the inventiveapparatus according to FIG. 2, FIG. 4 schematically shows in top view anenlarged apparatus which now comprises two plasma chamber stacks 20 aand 20 b as well as a transport area 23 _(T) and a load-lock chamber 30.With the help of the sequence of figures from 4 a to 4 e, a preferredoperation of such an inventive apparatus, especially for a PECVDtreatment process, shall be described.

In the operating phase, according to FIG. 4a, the workpieces arePECVD-treated in both the plasma chamber stacks 20 a and 20 b. To dothis, there is at least installed a pressure difference across apressure stage after shut-off of the handling openings 17 of the plasmachambers 1. The load-lock gate 32, according to FIG. 2a, is opened,whereas the load-lock gate 34 towards ambient is closed.

After termination of the treatment process, and as shown in FIG. 4b, thestacks 20 a and 20 b are unloaded by the transport arrangement 25,according to FIG. 2a. Thereby, in a preferred mode, the two stacks 20 aand 20 b are unloaded sequentially and the treated workpieces aredeposited within the magazine arrangement 36 in the load-lock chamber30. As will be described later, the magazine arrangement 36 comprisesmagazine slots, the number of which preferably according with at leastthe number of workpieces which may be simultaneously treated in theoverall apparatus. In other words, with two plasma chamber stacks,according to FIG. 4, there are preferably provided at least as manymagazine slots as plasma chambers at the two stacks.

According to FIG. 4c, the load-lock gate towards the transport are 23_(T) is then shut and the load-lock gate 34 opened. The treatedworkpieces deposited within the magazine arrangement 36 are now replacedby workpieces still to be treated. During this unloading cycle of themagazine arrangement 36, the plasma chambers 1 of the stacks areetched-cleaned, preferably by means of an RF plasma. To thereby preventthat cleaning gas and reaction products consisting of cleaning gas andetched-off material, etched-off from the plasma chambers beingetched-cleaned, penetrate into the transport area 23 _(T), in apreferred arrangement, as was described with the help of FIG. 3, thereis installed a pressure difference between plasma chambers 1 and thetransport area 23 _(T), pointing towards the inside of the plasmachambers 1.

To do this, there is introduced a neutral gas into the transport area 23_(T), as e.g. nitrogen, in such a manner that there results a pressuregradient dropping from the transport area 23 _(T) towards and into theplasma chambers 1. There is thereby prevented that cleaning dustpenetrates into the transport area 23 _(T). The chambers 1 themselvesare pumped during cleaning etching.

During this time-span, the magazine arrangement 36 has been loaded withworkpieces to be treated. These are then, in the next following stepaccording to FIG. 4d, distributed to the now cleaned plasma chambers ofthe staples.

Due to the cleaning etching step, the walls and the electrode surfacesof the plasma chambers 1 have been heated.

This heat is now, in a preferred mode according to step 4 e, used forpreheating the workpieces which are now loaded into the plasma chambers1. Because distribution of the workpieces, in cycle according to FIG.4d, is performed in vacuum, the heat conduction from the said parts,which have been heated by cleaning etching, is relatively low. Thus,after the workpieces to be treated have been loaded in the plasmachambers 1 and the latter are separated according to the description ofFIG. 3 at least by a pressure stage from the transport area 23 _(T),there is introduced a heat conducting gas, as e.g. hydrogen or helium,into the plasma chambers 1 with such a pressure that a significant heatconductance is initiated between the said heated parts of the plasmachambers 1 and the workpieces residing within the said chambers 1.

By means of such preheating of the workpieces, the workpieces, whichwere before stocked in normal atmosphere, are de-gased. After thispreheating they are now, according to FIG. 4a, treated in the plasmachambers 1, so, in a preferred mode, PECVD coated.

In the apparatus as shown, all the plasma chambers 1 are separatelypumped in a preferred embodiment. This especially during cleaningetching and during heating de-gasing of the workpieces.

As schematically shown in FIG. 5, for reactive treatment processes to beperformed, and especially for the preferred PECVD processes, accordingto FIG. 5a, all the plasma chambers 1 of at least one staple are fedfrom a central reactive gas feed. Thereby, it is ensured that all thechambers 1 are equally loaded with reactive gas. This is realized e.g.by feeding the gas departing from a buffer chamber 50 of relativelylarge volume to the chambers 1 via equal gas flow ducts 51, i.e.providing for equal flow resistances.

According to FIG. 5b, the chambers 1 are further, in a preferred mode,pumped from a central pump arrangement as all the chambers 1 of at leastone staple should be synchronously pumped.

The supply with electrical energy to the chambers 1 of at least onestaple is preferably realized in a most economic way from a centralgenerator unit. In the preferred case of treating the workpieces in anRF plasma, according to FIG. 5c, all the chambers 1 of at least onestaple are fed from a central RF generator with a centralized matchingnetwork and, if necessary, with additional matching networks for chamberspecific adjustment. This is shown in FIG. 5c by the respectiveinductions assigned to the respective chambers, wherewith different RFpower conditions may be adjusted for every chamber 1.

If the processes performed within the plasma chambers 1 shall bemonitored, open-loop controlled or negative feedback controlled, this,too, is preferably performed via a central unit. This central unit islinked to the different chambers 1, according to the occurring need, beit in the sense of multiplexing with a predetermined sequence ofconnection to the chambers, be it with a varying sequence, controlled bythe need at the different chambers 1.

This is schematically shown in FIG. 5d by means of an example, whichshows monitoring the processes in the chambers 1 by means of a centralplasma emission monitor which is sequentially connected to the differentchambers.

In FIG. 6 there is schematically shown a preferred construction of amagazine arrangement 36 in a magazine or load-lock chamber 30 accordingto FIG. 2a. The magazine arrangement 36 comprises a number of magazineslots 37, the number of which being preferably at least equal to thenumber of workpieces which may synchronously be treated in theapparatus. The number of magazine slots is thereby preferably the doubleof the number of workpieces which may be synchronously treated, i.e. thenumber of plasma chambers at the inventive apparatus. Thereby,feed-through of workpieces through the load-lock chamber issignificantly simplified. In the case where, according to FIG. 2a, thereis installed a relative vertical movement between resting surfaces forthe workpieces in the chambers 1 and the supports 27 by liftingmechanism 9 in the plasma chambers 1, as explained with the help of FIG.1, and thus the supports 27 do not perform vertical loading andunloading movements, then, and according to FIG. 6, the magazinearrangement 36 is preferably vertically movable as shown with the doublearrow ±V. Thereby, the workpieces may be lifted off or deposited from oron the supports 27.

Up to this point, there was described a novel apparatus concept as wellas its preferred operation, especially for RF-PECVD coating processes.

In the following, there will be described a novel method which also andespecially may be realized at the said described inventive apparatus.This method and accordingly apparative features to perform it results ina significant improvement of coating rate and coating quality at plasmacoating processes. The method and the respective apparatus features tobe described may be applied generally for plasma coating processes, beit DC, AC or AC+DC plasma processes, as they were defined before. Thefollowing description is nevertheless especially valid for reactive RFplasma enhanced coating processes as for RF-PECVD processes. They are,nevertheless, also valid e.g. for RF ion plating processes. Thereby, weunderstand, as was mentioned, under RF a frequency range between 1 and100 MHz.

Nevertheless, when we refer in the following description to such RFplasma enhanced reactive processes, this shall not be understood as thedescribed method restricted to such processes.

In FIG. 7 there is schematically shown a plasma chamber, e.g. of thekind as shown in the FIGS. 1 or 2. An areally extended RF electrode 60forms also an areally distributed gas injection arrangement, at leastfor a reactive gas G, which gas is injected into the plasma dischargespace PL. Opposite to the RF electrode 60 there is provided a workpiececarrier electrode 62, as known in this specific art.

With respect to electrical DC potential conditions, it is obvious to theman skilled in the art that the encapsulation walls 63 of the plasmachamber and/or the workpiece carrier electrode 62 may be deposited, asis common, on an electric reference potential, as e.g. on groundpotential. Nevertheless, the man skilled in the art effectively knowsall possibilities of biasing the different parts of the chamber toelectric DC potentials.

At a reactive plasma coating of workpieces, e.g. deposited on theworkpiece carrier electrode 62, there is formed dust within the plasmadischarge space. The density of this dust is depicted by P_(s). Dust inthe plasma discharge may originate from a multitude of sources, mainlyfrom the coating process itself, but also from mechanical frictionduring loading and unloading the chamber with workpieces. Principally,the dust density ps rises during a reactive coating process. This isshown at the bottom part of FIG. 7, purely qualitatively, by a steadilyrising characteristic (a) of dust density over time.

Without any counter-measures, the dust will start to precipitate out ofthe plasma discharge and will deposit on the surfaces within the reactorchamber exposed to the plasma discharge. Thereby, the growing up layeron the workpiece is contaminated with dust particles, which leads tolayer defects.

Additionally, the behaviour of the overall reactor is changed, whichleads to drifting off of the process. Today's production plants whichrealize dusty processes, therefore, do not lead to coatings with therequired low degree of defect nor do they reach the required low ratiobetween cleaning and production times, named equipment availability.

Up to now, the attempts to resolve these problems were to generate asfew as possible dust. Because this may not completely be prevented,still generated dust was removed from the coating area as completely aspossible. Thereby, one did not care about a resulting decrease ofcoating quality, as will be shown.

It was now recognized by the inventors of the present invention thatdust present in a plasma discharge, and thereby especially in an RFplasma discharge, significantly increases the coupling degree ofelectrical energy, and thereby especially of RF energy, to the plasmaand that principally the coating rate, and especially the coating rateof a reactive coating process, is significantly increased in a dustyplasma, especially in a dusty RF plasma.

Thus, up to now, preventing the formation of dust and removing dust fromthe plasma discharge led to non-exploitation of maximum possible coatingrate and process efficiency. The considerations with respect toimproving the efficiency and coating rate of a plasma coating process bydust are only valid so long as the dust density does not rise above athreshold value in the plasma discharge. If the dust density rises abovesuch limit value, dust particles may start to agglomerate to form largerdust particles, which will aggregate on the coating just being about tobe grown or having been deposited. Such aggregation must normally beprevented, especially in connection with semi-conductor production andthe production of flat active display screens.

Thus, the novel recognition bases on the fact that dust in a plasmadischarge area, especially in an RF plasma discharge area, as especiallyfor a reactive plasma enhanced coating process, should not be removed,but the dust density should be maintained below or at the most on apredetermined value p_(max). Thus, the number of dust particles per unitvolume and/or the largeness of such particles and therewith again thedust density and the distribution thereof are inventively controllablyadjusted. This adjustment is realized taking into consideration theresults of pre-experiments, whereat, for a specific treatment processconsidered, concise optimal dust density values and distributions in theplasma discharge space have been found. As an example, there is shown inFIG. 7 with the curve (b) a possible time course of power density whichis controllably aimed at.

This dust density control is generally realized, according to FIG. 7, bygenerating a dust particle transversal stream W_(ρ) in a controlledmanner by generating a transversal force field, so that excess dustparticles are carried out of the active coating area of-the plasmadischarge and are finally removed as the need occurs from the chamber.

According to FIG. 7, a preferred realization form of such a transversalforce field is to realize a transversal gas stream. This, again. isrealized by installing a transversal pressure gradient. As schematicallyshown, to do this, gas is laterally fed to the reactor chamber and gasis removed from that chamber opposite to its inlet. Additionally, orinstead of realizing a transversal pressure gradient, it is possible torealize transversal particle current by installing electrostaticalgradients and/or thermical gradients, so as to disable dust density torise in the coating effective area of the plasma discharge space above apredetermined value.

At the bottom of FIG. 7, the characteristic (c) qualitatively shows, asan example, the time course of controlled gas quantity {dot over(m)}_(G) inlet.

In spite of the fact that it is absolutely possible to determine bypre-experiments when, during the coating process, a transversal forcefield should be installed and how large it should be, so as to controlthe dust density in the discharge space, it is, in a preferred mode ofexecution, absolutely possible to measure, e.g. by means of lightreflection or absorption, as is schematically shown in FIG. 7 by thedetector 65, the instantaneous dust density and possibly itsdistribution in the plasma. The instantaneous value is then comparedwith a rated value F_(ρ) and the force field, which is, in FIG. 7, thepressure gradient, is then adjusted in a negative feedback controlledloop so that the dust density remains on a desired level. Because theplasma impedance is significantly influenced by the dust density, such anegative feedback control loop may also make use of a plasma impedancemeasurement to monitor the instantaneous dust density.

When a transversal gas stream is used to generate the described particlecurrent, the adjustment of such transversal gas stream is preferablydone by adjusting the amount of gas injected per time unit to the plasmadischarge space, as is shown schematically in FIG. 7 by adjusting walls67.

The force field which is used to remove excess dust particles from thecoating area may also intermittently be applied. This would mean, in thecase of FIG. 7, that a gas G_(s), which will be generally namedscavenger gas, which generates the transversal current W, is inlet in atimely pulsed manner.

As was already mentioned, this method has shown highly satisfyingresults, especially applied for reactive RF plasma coating processes.This, because such processes, and especially reactive processes,intrinsicly produce powder or dust in the plasma discharge.

If, according to FIG. 7, a scavenger gas G_(s) is used to realize thetransversal current W, preferably a neutral plasma working gas, as e.g.argon or helium, is used as scavenger gas or a gas which is noteffective for the coating deposition, as e.g. hydrogen. Using a gaswhich significantly contributes to the coating formation as a scavengergas, may influence coating deposition distribution in an undesiredmanner.

It is essential to recognize that dust or powder remains trapped withinthe plasma discharge as long as the plasma discharge is maintained.Thus, when a plasma treatment process or, more generally, the plasmadischarge shall be interrupted and one wants to prevent that the dusttrapped in the discharge settles in the treatment chamber, then one ofthe following procedures is proposed:

instead of reactive gas, a neutral gas is inlet, so that, when thecoating formation shall be stopped at a predetermined time, a furthercoating deposition is stopped. By maintaining the now neutral plasmaignited, the dust remains trapped in the plasma discharge and is sweptout. Thereby, additional formation of dust in the plasma, which is now aneutral plasma, is significantly reduced.

In the maintained reactive gas plasma discharge or in the just mentionedneutral discharge, the transversal force field is increased. In thepreferred mode of using a scavenger gas, the transversal stream ofscavenger gas is increased by increasing the amount of gas inlet pertime unit and/or increasing suctioning power at the gas removing port.

One may further continuously reduce the plasma discharge intensity, butthereby preventing extinction of the discharge. Thereby, the dusttrapping effect of the plasma discharge is steadily reduced, whichimproves sweeping out of the dust particles by the said transversalforce field.

By simultaneously reducing the discharge intensity and increasing thelateral pumping power and/or the amount of inlet scavenger gas per timeunit, a maximum sweep-out of the dust particles from the coating areaadjacent workpiece carrier electrode 62, according to FIG. 7, isreached.

Principally, the inventively applied transversal force field may beenhanced by operating the plasma discharge in a timely pulsed manner.Thereby, the average dust trapping effect of the plasma discharge isreduced and the controlled sweeping out of excess powder is simplified.This is not only valid for stopping the coating process, but also duringcoating.

It is essential that the plasma discharge is maintained up to the momentwhen the dust entrapped therein is at least substantially removed beforethe coating process is stopped.

This recognition leads to a further preferred mode of operation,according to which a plasma discharge is maintained in the plasmareactor chamber even then, when e.g. workpieces are loaded or disloadedto or from such reactor chamber.

A plasma discharge, which is, with respect to the reactive coatingprocess, ineffective, so in a gas which is neutral with respect to thecoating process, may be applied, with the target to etch-clean anuncoated or a coated workpiece or the plasma reactor chamber. E.g. ahydrogen plasma may be used for this target. It is thereby importantthat by such an etching plasma, also particles residing on theworkpieces are trapped from the discharge and may be swept out as wasdescribed.

Thus, e.g. at the end of a treatment process a hydrogen cleaning plasmamay be installed, e.g. during discharge and re-loading of the plasmachamber with workpieces. Because for etching dust in the plasmadischarge is only a disturbing factor, the transversal force field isthereby adjusted to its maximum effect.

By operating an RF plasma discharge for a reactive coating process bythe method which was described based on FIG. 7, for generatingα-Si-layers of defect-free quality and without dust deposition withinthe reactor, the coating rate, i.e. the amount of coating materialdeposited per time unit, was risen by a factor of about 2.5 andsimultaneously the inherent layer stress was reduced by a factor ofabout 2.5. Due to the increased deposition rate, the purity of theresulting layer was improved by a factor of approx. 2. This comparedwith a coating process in the same plasma reactor chamber, during which,by means of well-known techniques, it was attempted to keep the dustdensity minimal by selecting a process working point at low pressuresand at low power. When using the said known technique of dust densityminimalizing, the coating rate for α-Si-layers is smaller or equal 4Å/sec with layer stress larger than 5×10⁹dyn/cm². In opposition thereto,the inventive technique of dust density control leads to depositionrates of more than 10 Å/sec, so to rates of e.g. 33 Å/sec at layerstress smaller than 2×10⁹dyn/cm².

As was already mentioned, the preferred mode for realizing thetransversal force field is to instal a transversal gas stream, as hasbeen explained with the help of FIG. 7, across the plasma discharge.

In FIG. 8, five different variants a) to e) are shown to contribute toinstalling the said transversal force field, i.e. the said pressuregradient, by measures taken on the suction side, i.e. on the gasremoving side of the reactor.

According to FIG. 8a, pumping of gas from the treatment space or thedischarge space is realized through a narrow slit 69 in the wall of theplasma reactor chamber 1, which wall being electrically led on a definedpotential, so e.g. on ground potential. The width of the slit is so thatthe plasma discharge may not expand across the slit 69 and preferably isin the range of between 2 to 4 mm. In a preferred manner, thetransversal gas stream discussed with respect to FIG. 7 is significantlyco-realized by gas inlet through the scavenger gas inlet shown in FIG.7, which is also done at the embodiments according to FIG. 8b to 8 e.

According to FIG. 8b, a suctioning or pumping slit 71 is providedadjacent to the edge of electrode 60. Thereby, a further principle ispreferably followed. It was recognized that dust density is maximumthere where the electric field feeding the plasma discharge is maximum.This is, as well-known, at edges and spikes of equipotential surfaces.This is the reason why, according to FIG. 8b, pumping is realized bymeans of slit 71 adjacent to the corner of electrode 60, i.e. in an areawhere an increased dust density prevails because of the increasedelectric field strength.

In the embodiment according to FIG. 8c this concept is followed up inthat suctioning or pumping slits 71 a and 71 b are provided on bothsides adjacent the edges of the two electrodes 60 and 62.

The velocity of transversal gas stream is increased by the embodimentaccording to FIG. 8d which shows a steadily converging suctioning slit73. The increase of the said velocity becomes effective in a moreexpanded part of the plasma discharge area, so that efficiency oftransversal scavenger gas flow with respect to dust density control isimproved.

According to FIG. 8e, the wall part 75 of the plasma reactor chamber 1,wherein the suctioning slit 77 is provided, is operated electrically onfloating potential. Thereby it is reached that the electrostaticalpotential barrier, which has to be overcome by the powder particles asthey are extracted from the discharge, is lowered. This because theelectric potential of the wall part 75, operated on floating potential,will assume an intermediate value between the potentials of theelectrodes 60 and 62.

A further possibility which is quite obvious is to provide across thesuctioning slit a grid. Thereby, the opening of the slit and thus thepump-effective cross-section of the pumping pipe may be enlarged,without that the plasma discharge penetrates through the slit.

It is further evident that the features of the gas removingarrangements, according to the embodiments of FIG. 8, may be combined.

Looking back to the novel apparatus configuration with plasma chamberstaples, there is shown, in FIG. 9, how at such an apparatus, andconsidering the just described novel approach, charging and unloadingthe plasma chambers 1 is preferably carried out.

According to FIG. 9a, a workpiece 31 (see FIG. 2a) is deposited on therods 13 of a lifting mechanism 9. According to FIG. 9b, as workpieces 31have been introduced into the chambers 1 and a pressure difference maybe installed between chamber 1 and transport area 23 _(T), there isignited a neutral plasma in the plasma chamber 1, after such a pressuredifference has been installed between the inside of the plasma chamber 1and the transport area 23 _(T) according to FIG. 3, by means of theshutting-off plates 41. Such a plasma is maintained during the stepsaccording to FIGS. 9b and 9 c. A nonreactive gas, so e.g. argon and/orhydrogen, is inlet to the reactor chamber as shown. Thereby, and as wasdescribed, the workpiece 31 is also heated up, so e.g. for its degasing.Suctioning at A is active.

Dust which has e.g. formed during mechanical movement of the workpiece31, according to FIG. 9c, is trapped in the neutral plasma and isremoved by means of the neutral gas transversal stream at suctioningport A. As soon as, according to FIG. 9d, the workpiece 31 has beenlowered in its treatment position, the reactive gas inlet is initiatedin a preferred manner through the gas inlet shower formed by the RFelectrode 62, as well as through the lateral scavenger gas inlet asshown. During the coating process following up the dust density in theplasma discharge is not minimized, but is, as was described with thehelp of FIG. 7, controllably adjusted so as not to grow above apredetermined level.

After termination of the coating process, and according to FIG. 9e, thetransversal stream of reactive gas is increased or there is injected aneutral gas or there is switched onto a neutral plasma discharge (notshown), as was earlier described. It is important that also duringlifting up of the coated workpieces, according to FIG. 9f, there ismaintained a dust trapping plasma discharge, be it a neutral plasmadischarge or the reactive plasma discharge, latter in the case wherecoating process needs not to be terminated at a well-defined moment.

According to FIG. 9g, thereafter, the workpiece 31 is removed from theplasma chamber 1. In this operating phase, as well as possibly in thatshown in FIG. 9a, but especially in that of FIG. 9b, it is preferred toalso maintain a plasma discharge, not a reactive plasma discharge, but aneutral plasma discharge, especially a hydrogen plasma discharge. This,on one hand, for further trapping dust particles, and, on the otherhand, for cleaning-etching the inside of plasma reactor chamber 1.

As is shown in the FIGS. 9a and 9 g, there is introduced thereforehydrogen gas and, on the other hand, gas is removed by suctioning, sothat dust trapped in the plasma is removed from the reactor chamber ascompletely as possible in the respective operating phases.

Additionally to the described gas suctioning ports, co-installing thetransversal gas stream, additional suctioning openings may be provided,so e.g. along the electrode 60, and distributed as the reactive gasinlet openings there-along. By specific layout of the distribution ofsuch gas inlet and outlet openings, the homogenity of the coating alongthe workpiece surfaces, thus the resulting thickness uniformity of thefilm deposited, may possibly optimized.

As was mentioned above, the described inventive apparatus and thedescribed inventive method are especially suited for the production offlat active display screens.

What is claimed is:
 1. A method for manufacturing flat active displayscreens by a vacuum treatment facility, comprising: simultaneouslyintroducing a set of horizontally oriented screen workpieces spaced oneabove the other into at least one input loadlock; simultaneouslytransporting said horizontally oriented screen workpieces remainingspaced one above the other out of said at least one loaded loadlocktowards treatment stations; simultaneously treating said screenworkpieces at said treatment stations, said workpieces remaining spacedone above the other and horizontally oriented; simultaneouslytransporting said workpieces from said treatment stations to at leastone output loadlock while still remaining spaced one above the other andhorizontally oriented; and removing said screen workpieces from saidoutput loadlock.